Washing liquid for low temperature apparatus, particularly gas liquefaction and fractionating apparatus



fractionation of: gas mixtures.

Patented May 11, 1954 APPARATUS, PARTICULARLY GAS: LIQUE- .FACTION .AND.FRACTIONATING APPA- RATUSL iKarl Bit'cliner,.Duisliurg-Hamborn, andHeinrich Spiiiinken, 'Oberhausen-Holten; Germany, "assignors to:

Ruhrchemie Aktiengesellschaft,

Obethausen-Holten, Germany, a corporation oi- Germany -N'oi Drawing;Application November-16, 1950,

.seriar No. 196,088

I This invention relates to improvements in washing liquids for lowtemperature apparatus.

Countercurrent heat exchangers, washing columns, separating columnsandfractionating apparatus are used. for .the liquefaction and Intheseknown apparatuses, the gas constituents which have beenzrisolated. inliquid or gas form remove. the

'heat from the gases which are tobeprocessed.

It is in this manner that air and coke ovengas arenliquefiedto alarge..extent. and split into their: individual. constituents. Even withgood preliminary purification, the entering gases still containquantities of gaseous impurities such as carbon-dioxide, nitrogen.oxide, acetylene, etc.

.These impuritiessolidify at. the. low temperatures present, ie.temperatures. as low as -.-180 C. andthereforedeposit. on the heatvexchange 1 surfaces. 7

If these deposits were allowed to remain, they would clog the apparatus.In order to avoid this; rclogging these: deposits." must. be; removedfrom timewto time. This removal may be accomplishedby thawing theliquefaction and fractionating apparatus. The thawing" procedure,however, entails-greatdanger inthat if acetylene or nitrogen oxides havedeposited in solid .form on the heat exchange surfaces. duringf'theoperation, explosions may occur: Due to the ever presentdanger of theseexplosions, it is preferable not to use the thawing procedure.Accordingly, it is preferable to wash the heat exchange and separatingapparatuses with solvents which have a low solidification point, andalso dissolve chemicals such as caustic soda. By use of these chemicals,the solidified gas constituent can be rendered harmless byneutralization or other reaction.

It is advantageous to introduce these solvents into the apparatus at alow temperature so that there is no unfavorable heat exchange in theapparatus when the solvents are added. A solution containing 90% byvolume of methanol and 10% by volume of sodium hydroxide solution, thesodium hydroxide solution containing 7% by weight of NaOH (70-80 gramsNaOH in 1000 cc. of this solution), has been used in the past. Mixturesof ethyl alcohol and sodium hydroxide solution or potassium hydroxidesolution have also been used for this purpose.

The solidification point of the mixture consisting of 90% by volume ofmethanol, 10% by volume of sodium hydroxide solution of the abovementioned concentration is approximately between -1l0 and C. The heatexchanges and separating apparatus assume temperatures of about C. andeven lower upon splitting 'up of thecoke-oven gas. When washing thisapparatus in cold condition with the said mixture of. 90% by volume ofmethanol and 10% by volume of sodium. hydroxide solution the danger thatthe solvent will partiallysolidify in theapparatus andv that thesolidified gas con stituentsr cannot be completely removed is=alwayspresent. If this occurs, a new washing becomes.

necessary after a very short period ofv operation. In accordance withthe invention, we have found a new washing liquid which overcomes. thesedifliculties. Accordingto the. pres.- ent invention, a washing solutionconsistingv of an alcoholic alkali solution which. contains hydrocarbonsof a low boiling point, is used. We have found that it is preferable touse a..mix.- ture of approximately 90% by volume. of methanol and 10% byvolume of dilute sodiumv hydroxide solution, said. solution containingabout 7% by weight .NaOH, which. has. been previously saturated with lowboiling hydro- .carbonsand preferably with hydrocarbons with ,amolecular size of less thanv 5 carbonv atoms. This new mixture has apour point of -.145 CI, so that when it is used as a washing solvent itno longer. solidifies or becomes viscous within the "low temperatureapparatus which is being washed out. The washing liquid in accordancewith the invention may, for example, be used as follows.

Coke oven gas at a temperature of 40 C. are cooled in 2 counter-currentapparatuses by returning gases to a temperature of 150 C. Upon cooling,the ethylene in the gases is separated as a liquid. The separatedethylene is in part subjected to a decreased pressure and in part cooledto still a lower temperature in a special counterfiow apparatus withliquid nitrogen. The gas mixture from which the ethylene has beenremoved then flows through another counter-current heat-exchange afterwhich it is rectified in an evaporator having a washing columnconnecting to it.

After a relatively long period of operation, all the heat-exchangingapparatuses (countercurrent apparatuses) contain a thin layer ofsolidified gas constituents. In order to remove these deposits theapparatuses are filled, without prior thawing, with the new washingliquid in accordance with the invention. With this new washing liquidthere is now no danger that local solidification or thickening of thisliquid will prevent its further penetration to all parts of theapparatus.

It can thus be seen that one great advantage among other in using thenew washing liquid in accordance with the present invention consists inthe fact that the separating apparatus treated is capable of operationfor 'a considerably longer period of time than before, for instance, upto 2,000 hours of operation. Before the use of the new washing liquid,washing had to be effected in general after about 1000 hours ofoperation.

The liquid, in accordance with the invention, is not limited to thewashing or purifying gas liquefaction and fractionation installationsbut may be used anywhere where a mixture of alcohol and alkali solutionwith a low pour point is desired. With an increase in the content of theaqueous alkali solution, and in particular sodium hydroxide solutions,the pour points of the alcoholic alkali solution is considerablyincreased.

The addition of the hydrocarbon considerably lowers the pour point, forexample, a mixture containing 90% by volume of ethyl alcohol and byvolume of potassium hydroxide solution (7% by weight KOH) solidifies at130 C. If this mixture is saturated with gaseous hydrocarbons, as forexample methane, ethane, propane or the corresponding olefins ormixtures thereof, the pour-point drops to -145 C. so that such analcoholic potassium hydroxide solution can be used at considerably lowertemperatures than where heretofore possible.

Similar reactions also apply to propyl alcohol. A mixture of 90%isopropyl alcohol and 10% sodium hydroxide solutions of 7% concentrationsolidifies at -105 C. After saturation with gasol i. e. hydrocarbonscontaining 3 and 4 carbon atoms in the molecule, the pour-point islowered to 120 C.

Also other aliphatic alcohols containing 1 to 3 carbon atoms in themolecule may be used instead of methyl alcohol. The aqueous alkali lyemay thus be mixed, for example, also with ethyl alcohol or propylalcohol. Among the propyl alcohols, the ordinary propyl alcohol may mostpreferably be used; also isobutyl alcohol is in part still suited asalcoholic component.

The alkali concentration of the aqueous constituents of the solvent inaccordance with the invention should not be so high that solid hy- 4drates of the alkali hydroxides form when cooling down to for instance-120 C. Therefore, the concentration of the aqueous alkali hydroxidesolutions should generally not be in excess of 10% of alkali hydroxide.

In order to lower the solidification point of the alcoholic alkali lye,paraflinic or olefim'c hydrocarbons containing 1 to 4 carbon atoms inthe molecule may be used. Methane, ethane, ethylene, propane, propylene,butane, and butylene may thus be employed for the method in accordancewith the invention. A hydrocarbon mixture which may be separated, forinstance, from coke oven gas and which is known in the market as gasolis very well suited. It contains on an average 19% propane, 45%propylene, 5%

ethane. 2% ethylene, 3% butane, 13% butylene,

and 13% by volume of other constituents (N2, 02, CO2, CH4) A solutionwhich consists of by volume of methanol and 10% by volume of an aqueoussodium hydroxide solution containing 7% by Weight of NaOH absorb pervolume approximately 400 to 500 cc. of this hydrocarbon mixture at atemperature of approximately 18 C.

It can now be seen from the above that the invention lies in compositionof a new washing liquid containing an alcohol, an alkali solution, andlow boiling hydrocarbons.

We claim:

Washing liquid for low-temperature apparatus comprising a mixture ofabout 90% of an alcohol selected from the group consisting of methanoland ethanol, and about 10% of an aqueous alkali solution of about 7%concentration of a member selected from the group consisting ofpotassium hydroxide and sodium hydroxide substantially saturated with anormally gaseous hydrocarbon having between 1 and 4 carbon atoms in itsmolecule.

References Cited in the file of this patent UNITED STATES PATENTSSwitzerland July 1, 1933

